To calculate a valid block for a repo:
nix-shell -p nix-prefetch-github
nix-prefetch-github $owner $repo --nix
For example:
[nix-shell:~/Developer]$ nix-prefetch-github kidonng nix.fish --nix
let
pkgs = import <nixpkgs> {};
in
pkgs.fetchFromGitHub {
owner = "kidonng";
repo = "nix.fish";
rev = "ad57d970841ae4a24521b5b1a68121cf385ba71e";
hash = "sha256-GMV0GyORJ8Tt2S9wTCo2lkkLtetYv0rc19aA5KJbo48=";
}